title: Platelet-derived growth factor-A regulates lung fibroblast S-phase entry through p27(kip1) and FoxO3a.
abstract: BACKGROUND
  Secondary pulmonary alveolar septal formation requires platelet derived growth factor (PDGF-A) and platelet derived growth factor receptor-alpha (PDGFRα), and their regulation influences alveolar septal areal density and thickness. Insufficient PDGFRα expression in lung fibroblasts (LF) results in failed septation.
  
  
  METHODS
  Mice in which the endogenous PDGFRα-gene regulates expression of the green fluorescent protein were used to temporally and spatially track PDGFRα-signaling. Transition from the G₁/G₀ to the S-phase of the cell cycle was compared in PDGFRα-expressing and non-expressing LF using flow cytometry. Laser scanning confocal microscopy was used to quantify p27(kip1) and forkhead box "other" 3a (FoxO3a) in the nuclei of alveolar cells from mice bearing the PDGFRα-GFP knock-in, and p27(kip1) in mice with a conditional deletion of PDGFRα-gene function. The effects of PDGF-A on the phosphorylation and the intracellular location of FoxO3a were examined using Western immuoblotting and immunocytochemistry.
  
  
  RESULTS
  In neonatal mouse lungs, entry of the PDGFRα-expressing LF subpopulation into the S-phase of the cell cycle diminished sooner than in their non-expressing LF counterparts. This preferential diminution was influenced by PDGFRα-mediated signaling, which phosphorylates and promotes cytoplasmic localization of FoxO3a. Comparative observations of LF at different ages during secondary septation and in mice that lack PDGFRα in alveolar LF demonstrated that nuclear localization of the G₁ cyclin-dependent kinase inhibitor p27(kip1) correlated with reduced LF entry into S-phase.
  
  
  CONCLUSIONS
  Nuclear localization of FoxO3a, an important regulator of p27(kip1) gene-expression, correlates with diminished proliferation of the PDGFRα-expressing LF subpopulation. These mechanisms for diminishing the effects of PDGFRα-mediated signaling likely regulate secondary septal formation and their derangement may contribute to imbalanced fibroblast cell kinetics in parenchymal lung diseases.
authors.affiliation: Department of Veterans Affairs Research Service, University of Iowa Carver College of Medicine, Iowa City, IA, USA. stephen-mcgowan@uiowa.edu
authors.forename: Stephen E
authors.initials: SE
authors.lastname: McGowan
authors.forename: Diann M
authors.initials: DM
authors.lastname: McCoy
mesh_terms: D000818:Animals; D002467:Cell Nucleus / Q000378:metabolism; D002478:Cells, Cultured; D050760:Cyclin-Dependent Kinase Inhibitor p27 / Q000378:metabolism*; D005347:Fibroblasts / Q000166:cytology* / Q000378:metabolism*; D000071316:Forkhead Box Protein O3; D051858:Forkhead Transcription Factors / Q000378:metabolism*; D005786:Gene Expression Regulation / Q000502:physiology; D008168:Lung / Q000166:cytology* / Q000378:metabolism*; D051379:Mice; D008822:Mice, Transgenic; D010982:Platelet-Derived Growth Factor / Q000378:metabolism*; D059807:S Phase Cell Cycle Checkpoints / Q000502:physiology; D015398:Signal Transduction / Q000502:physiology
chemical_list: D000071316:Forkhead Box Protein O3; D051858:Forkhead Transcription Factors; C474033:FoxO3 protein, mouse; D010982:Platelet-Derived Growth Factor; C075278:platelet-derived growth factor A; D050760:Cyclin-Dependent Kinase Inhibitor p27